Analog circuit and display device using the same

ABSTRACT

The invention provides an analog circuit that decreases an effect of variation of a transistor. By flowing a bias current in a compensation operation, a voltage between the gate and source of the transistor to be compensated is held in a capacitor. In a normal operation, the voltage stored in the compensation operation is added to a signal voltage. As the capacitor holds the voltage according to the characteristics of the transistor to be compensated, the effect of variation can be decreased by adding the voltage stored in the capacitor to the signal voltage. Further, an analog circuit which decreases the effect of variation can be provided by applying the aforementioned basis to a differential circuit, an operational amplifier and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an analog Circuit technology. Morespecifically, the invention relates to a circuit technology to decreasean effect of variation of current characteristics of transistors.

2. Description of the Related Art

In recent years, a display device having a thin film transistor (TFT)formed on a glass substrate is widely used. For example, a liquidcrystal display (LCD) having in each pixel a TFT formed by usingamorphous (non-crystalline) silicon is in widespread use in a notebookpersonal computer or a portable device and the like.

The TFT formed by using amorphous silicon, however, has low mobility.Therefore, it cannot flow much current. In view of the aforementioned, aTFT formed by using polycrystalline silicon is formed on a glasssubstrate instead. By using the polycrystalline silicon TFT that hashigh mobility, it is possible to integrate a driver circuit on the glasssubstrate. A driver circuit is typically implemented with a digitalcircuit, however, it is also under study to implement an analog circuitas well aiming at the realization of System-on-Panel in which variouscircuits are mounted on a glass substrate.

A structure of a source follower circuit as an example of the analogcircuit is described now. FIG. 21 shows a circuit diagram of a sourcefollower circuit. An input voltage Vi is inputted to a gate terminal4308 of a transistor TR1. A bias voltage Vb is applied to a gateterminal 4309 of a transistor TR2. A voltage between the gate and sourceof the transistor TR1 is denoted as Vgs1. It is assumed for simplicitythat a potential of a power supply on the low potential side (Vss) is 0V. Then, a voltage of a source terminal 4310 of the transistor TR1 (anoutput voltage Vo) satisfies the following formula (1).Formula 1 V _(o) =V _(i) −V _(gs1)  (1)

It is also assumed for simplicity that current characteristics and sizes(gate length L and gate width W) of the transistors TR1 and TR2 areidentical here. The transistors TR1 and TR2 are connected in series,therefore, the same amount of current flows through each transistor. Inthe case where the transistors TR1 and TR2 both operate in a saturationregion, the voltage Vgs1 between the gate and source of the transistorTR1 is equal to a voltage between the gate and source of the transistorTR2, that is the bias voltage Vb. Therefore, the following formula (2)is satisfied.Formula 2 V _(o) =V _(i) −V _(b)  (2)

Although the sizes (gate length L and gate width W) of the transistorsTR1 and TR2 are designed to be identical, actual sizes often vary whenfabricated. Further, variation of gate insulating films in thickness orvariation of crystallinity of channel formation regions lead tovariation of the current characteristics of the transistor, for examplethreshold voltage or mobility.

It is assumed here as an example that a threshold voltage of thetransistor TR1 is 2 V and that of the transistor TR2 is 3 V because ofthe variation. It should be noted that the transistor flows a current,which corresponds to a value that deducted a threshold voltage from avoltage between the gate and source thereof. In order that thetransistor TR1 may flow the same amount of current as that flowingthrough the transistor TR2, a voltage between the gate and source of thetransistor TR1 becomes 1 V lower because the threshold voltage thereofis 1 V lower. As a result, the output voltage. Vo becomes 1 V higher,which is defined by Formulas 1 and 2 as compared to the case where thethreshold voltage of the transistors TR1 and TR2 are equal.

As described above, the output voltage Vo varies when the currentcharacteristics or the sizes of the transistors TR1 and TR2 vary.

In view of the aforementioned, a technology for making compensation forthe variation is suggested. For example, a source follower circuit inwhich variation of transistors is compensated is reported (refer toNon-Patent Document 1).

FIG. 24 shows a circuit diagram of the aforementioned source followercircuit. The operation thereof is described now. First, switches 4401,4406, and 4404 are turned ON among switches 4401 to 4406. Note that aswitch becomes conductive when turned ON. An input voltage Vi is appliedto an input terminal 4407. Subsequently, the switches 4401 and 4406 areturned OFF and the switch 4402 is turned ON. Then, a first offsetvoltage is stored in a capacitor 4409. Next, the switches 4402 and 4404are turned OFF and the switch 4403 is turned ON. Then, a second offsetvoltage is stored in a capacitor 4410. As a result of the aforementionedoperations, variation of the output voltage Vo is compensated.

Non-Patent Document 1

Euro Display 2002: p.831: LN-4: A 3.8 inch Half-VGA Transflective ColorTFT-LCD with Completely Integrated 6-bit RGB Parallel Interface Drivers

When making compensation by using the aforementioned source followercircuit in FIG. 24, quite a few steps are required. That is,compensation is made by repeating ON/OFF of the switches 4401 to 4406.Therefore, it takes long time for making compensation until the originaloperation starts.

Additionally, a lot of switches and capacitors are required in theaforementioned source follower circuit, which occupy wider area for thelayout and may lower the yield.

In other analog circuits as well as the source follower circuit, anormal operation may not be performed or an output may vary when currentcharacteristics of transistors and the like vary.

In view of the aforementioned problems, the invention provides anelectric circuit that suppresses an effect of variation ofcharacteristics of transistors. More specifically, the inventionprovides an electric circuit for processing analog signals that allows adesired operation while suppressing an effect of variation ofcharacteristics of transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a differential circuit ofthe invention.

FIG. 2 is a diagram showing an operation of a differential circuit ofthe invention.

FIG. 3 is a diagram showing an operation of a differential circuit ofthe invention.

FIG. 4 is a diagram showing an operation of a differential circuit ofthe invention.

FIG. 5 is a diagram showing an operation of a differential circuit ofthe invention.

FIG. 6 is a diagram showing an operation of a differential circuit ofthe invention.

FIG. 7 is a diagram showing a configuration of a source follower circuitof the invention.

FIG. 8 is a diagram showing a configuration of a switching amplifiercircuit of the invention.

FIG. 9 is a diagram showing a configuration of a display device of theinvention.

FIG. 10 is a diagram showing a configuration of a display device of theinvention.

FIG. 11 is a diagram showing a configuration example of a signal drivercircuit of the invention.

FIG. 12 is a diagram showing an example of a configuration of a signaldriver circuit of the invention.

FIG. 13 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 14 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 15 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 16 is a diagram showing a configuration of a cascode circuit of theinvention.

FIG. 17 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 18 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 19 is a diagram showing a configuration of a source followercircuit of the invention.

FIG. 20 is a diagram showing a layout of a source follower circuit ofthe invention.

FIG. 21 is a diagram showing a configuration of the conventional sourcefollower circuit.

FIG. 22 is a diagram showing a configuration of a basic circuit of theinvention.

FIG. 23 is a diagram showing a configuration of a basic circuit of theinvention.

FIG. 24 is a diagram showing a configuration of a conventional sourcefollower circuit.

FIG. 25 is a diagram showing a configuration of a differential amplifiercircuit of the invention.

FIG. 26 is a diagram showing a configuration of a differential amplifiercircuit of the invention.

FIG. 27 is a diagram showing a configuration of a differential amplifiercircuit of the invention.

FIG. 28 is a diagram showing a configuration of a differential amplifiercircuit of the invention.

FIG. 29 is a diagram showing a configuration example of an operationalamplifier of the invention.

FIG. 30 is a diagram showing a configuration example of an operationalamplifier of the invention.

FIG. 31 is a diagram showing a configuration example of an operationalamplifier of the invention.

FIG. 32 is a diagram showing a configuration of a common sourceamplifier circuit of the invention.

FIG. 33 is a diagram showing an operation of a common source amplifiercircuit of the invention.

FIG. 34 is a diagram showing an operation of a common source amplifiercircuit of the invention.

FIG. 35 is a diagram showing a configuration of a common sourceamplifier circuit of the invention.

FIGS. 36A to 36H are views showing electric devices to which theinvention is applied.

SUMMARY OF THE INVENTION

The invention employs an analog circuit having a configuration describedbelow in view of the aforementioned problems.

The invention provides an analog circuit including a first transistor, afirst capacitor, a first switch, a first terminal, a second terminal, asecond transistor, a second capacitor, a second switch, a thirdterminal, and a fourth terminal. A gate terminal of the first transistorand one terminal of the first capacitor are electrically connected and agate terminal of the second transistor and one terminal of the secondcapacitor are electrically connected and a source terminal of the firsttransistor and a source terminal of the second transistor areelectrically connected. The first terminal and one terminal of the firstcapacitor are electrically connected via the first switch, and the thirdterminal and one terminal of the second capacitor is electricallyconnected via the second switch. The analog circuit of the inventionfurther includes a unit to connect the other terminal of the firstcapacitor to either the second terminal or a source terminal of thefirst transistor electrically, and a unit to connect the other terminalof the second capacitor to either the fourth terminal or a sourceterminal of the second transistor electrically.

There are two operating states in the analog circuit of theaforementioned configuration. One is a compensation operation and theother is a normal operation. In the compensation operation, data forcompensating the effect of variation of characteristics of transistorsis obtained. Meanwhile in the normal operation, the data obtained in thecompensation operation is added to an input signal to perform theoriginal operation of the analog circuit. With the data obtained in thecompensation operation added to the input signal in the normaloperation, the effect of variation is decreased in the normal operation.

Further, the data obtained in the compensation operation is stored andused in the normal operation later. In this manner, a compensationoperation does not have to be performed every time the normal operationis performed.

Next, a connection of a circuit in each operation is described.

FIG. 22 shows a connection of a circuit in the compensation operation. Acapacitor 104 is disposed between the gate terminal and the sourceterminal of the transistor TR1. One terminal of the capacitor 104 andthe gate terminal of the transistor TR1 are electrically connected, andthe other terminal of the capacitor 104 and the source terminal of thetransistor TR1 are electrically connected. As each terminal iselectrically connected, a switch in ON-state, a passive element, or anactive element may be disposed on the wirings between the terminals. Itshould be noted that referred to as a connection in this specificationmeans an electrical connection. Therefore, there may be other elements(for example, other elements, a switch or the like) which enableelectrical connection between the predetermined connections in theconfiguration disclosed in the invention. Further, each of the gateterminal, the drain terminal, and the source terminal of the transistorTR1 is electrically connected to other elements (such as a switch, anactive element such as a transistor, or a passive element) or wirings.

The aforementioned connection corresponds to the connection in which thegate terminal of the first transistor and one terminal of the firstcapacitor are connected, the first terminal and one terminal of thefirst capacitor are connected, the other terminal of the first capacitorand the second terminal are not connected, and the other terminal of thefirst capacitor and the source terminal of the first transistor areconnected.

In the aforementioned configuration, a current having a certain valueflows between the drain and source of the transistor TR1. Note that thevalue may be an arbitrary value including zero. The capacitor 104 storesa voltage Vgs between the gate and source of the transistor TR1 throughwhich the aforementioned current is flowing. The voltage Vgs between thegate and source of the transistor TR1 is dependent on the currentflowing between the drain and source thereof. Therefore, when thecurrent characteristic or the size of the transistor TR1 varies, thevoltage Vgs between the gate and source of the transistor TR1 variesaccordingly. It should be noted that the voltage Vgs between the gateand source of the transistor TR1 is dependent on the current flowingbetween the drain and source of the transistor TR1 even when thecharacteristic of the transistor TR1 varies.

In this manner, data for compensating for the effect of variation, thatis the voltage between the gate and source of the transistor TR1 isobtained in the compensation operation.

FIG. 23 shows a connection of the circuit in the normal operation. Thecapacitor 104 is disposed between the gate terminal of the transistorTR1 and an input terminal 108. One terminal of the capacitor 104 and thegate terminal of the transistor TR1 are electrically connected, and theother terminal of the capacitor 104 and the input terminal 108 areelectrically connected. An input voltage Vi is applied to the inputterminal 108. The capacitor 104 stores a charge obtained in thecompensation operation. Therefore, a voltage in which the voltage storedin the capacitor 104 is added to the input voltage Vi is applied to thegate terminal of the transistor TR1.

The aforementioned connection corresponds to the connection in which thegate terminal of the first transistor and one terminal of the firstcapacitor are connected and the first terminal and one terminal of thefirst capacitor are not connected. The other terminal of the firstcapacitor and the second terminal are connected and the other terminalof the first capacitor and the source terminal of the first transistorare not connected.

In this manner, the gate terminal of the transistor TR1 is not appliedthe input voltage Vi as it is, but the voltage in which the voltagestored in the capacitor 104 is added to the input voltage Vi. Thevoltage stored in the capacitor 104 is dependent on the currentcharacteristic and the size of the transistor TR1. That is, the voltagestored in the capacitor 104 changes according to the variation of thecurrent characteristic and the size of the transistor TR1. Thus, aneffect of variation of the transistor TR1 can be decreased.

By making the aforementioned compensation to each transistor, variationof the circuit as a whole can be compensated. That is, by makingcompensation to the first transistor, the second transistor, or varioustransistors configuring the circuit, variation can be compensated.

Note that disposing a switch between certain terminals allows theelectrical connection as shown in FIG. 22 in the compensation operation,and the electrical connection as shown in FIG. 23 in the normaloperation. Several numbers of such switches may be disposed.

The transistor TR1 is an n-channel transistor in FIGS. 22 and 23,however, the invention is not exclusively limited to this and it may bea p-channel transistor as well. A p-channel transistor can be used asthe transistor TR1 in the compensation operation by disposing thecapacitor 104 between the gate and source thereof.

The compensation operation may be performed at least once beforeperforming the normal operation. That is, the normal operation can beperformed as long as an appropriate voltage is held in the capacitor104. However, the charge stored in the capacitor 104 may change due to anoise or a leak current. In that case, a compensation operation isperformed once again before the charge stored in the capacitor 104changes drastically.

As described above, only once of the compensation operation can decreasethe effect of variation of characteristics of transistors in thesubsequent normal operation. Thus, drive timing is not made complicatedand a simple operation can be obtained.

Furthermore, only the capacitor 104 may be provided as a capacitor, anda few numbers of switches may be provided, which occupies a small areafor the layout. As a result, it is efficient for downsizing whilepreventing the drop in manufacturing yield.

It should be noted that the transistor in the invention may be anytransistor formed by using any material, unit, or manufacturing method.For example, it may be a thin film transistor (TFT). The TFT may beformed by using a non-crystalline (amorphous), polycrystalline, or asingle crystalline semiconductor layer. It may be a transistor formed ona single crystalline substrate, a SOI substrate, a plastic substrate, ora glass substrate. It may be a transistor formed by using organiccompounds or carbon nanotube. It may also be a MOS transistor or abipolar transistor.

The invention of the aforementioned configuration provides an analogcircuit that includes a unit for supplying current which is electricallyconnected to the source terminal of the first transistor.

In this manner, by providing the unit for supplying current, a biasvoltage of the analog circuit can be determined.

Further, the invention of the aforementioned configuration provides ananalog circuit that includes a unit for interrupting the current flowingto the first transistor and a unit for interrupting the current flowingto the second transistor.

According to the aforementioned configuration, compensation operationcan be performed separately to the first transistor and the secondtransistor.

Further, the invention of the aforementioned configuration provides ananalog circuit in which the first terminal and the second terminal areelectrically connected and the third terminal and the fourth terminalare electrically connected.

According to the aforementioned configuration, a wiring for supplying avoltage to the first terminal and the third terminal can be removed.

DETAILED DESCRIPTION OF THE lNVENTION

Embodiment Mode 1

The invention can be applied to a variety of circuits such as an analogcircuit including a differential circuit, an amplifier circuit, or anarithmetic circuit represented by an operational amplifier. In thisembodiment mode, the differential circuit to which the invention isapplied is described as an example.

FIG. 1 shows a circuit configuration of a differential circuit to whichthe invention is applied. In a conventional differential circuit, atransistor TR21 which operates as a power supply and determines a biasof the circuit is disposed, and the source terminal of a transistor TR11for a differential operation and the source terminal of a transistorTR12 are connected to the drain terminal of the transistor TR21. Thedrain terminal of the transistor TR11 is connected to a power supply onthe high potential side (Vdd) via a load 1812 and the like, and thedrain terminal of the transistor TR12 is also connected to the powersupply on the high potential side (Vdd) via a load 1813 and the like.

On the other hand, the differential circuit to which the invention isapplied includes switches 1801 to 1811 and capacitors 1812 and 1813additionally.

When using a transistor as a switch, the conductivity of the transistoris not particularly limited as it operates simply as a switch. However,in the case where an off current is preferably small, a transistorhaving a conductivity of a smaller off current is preferably used, forexample as switches connected to the capacitors 1812 and 1813. One ofthe transistors of a small off current is a transistor provided with anLDD region. It is preferable that an n-channel transistor be used in thecase where a potential of the source terminal to operate as a switch isclose to the power supply on the low potential side (Vss, Vgnd, 0V andthe like), while a p-channel transistor is preferably used in the casewhere the potential of the source terminal thereof is close to the powersupply on the high potential side (Vdd and the like). This is because anabsolute value of the voltage between the gate and source can be large,which makes the operation as a switch easy. Note that a CMOS circuit mayalso be formed by using both n-channel and p-channel transistors.

A switch used in the invention may be any switch such as an electricalswitch or a mechanical switch. That is, it may be anything as far as itcan control a current. It may be a transistor, a diode, or a logicCircuit configured with them.

The operation of the differential circuit in FIG. 1 is described withreference to FIGS. 2 to 6. First, a compensation operation is performed.At that time, the compensation operation may be performed to thetransistors TR11 and TR12 at the same time. However, the transistor TR21is the only transistor that operates as a power supply. It is consideredthat a compensation operation is performed with higher precision byusing the same transistor. Therefore, the first compensation operationis performed by using the transistors TR11 and TR21, and then the secondcompensation operation is performed by using the transistors TR12 andTR21. Note that these compensation operations can be performed inreverse order.

As shown in FIG. 2, a compensation operation is performed by using thetransistors TR11 and TR21. At this time, current to be supplied to thetransistor TR21 is controlled to flow to the transistor TR11 but not tothe transistor TR12 because the current flowing to the transistor TR12causes a possible error. Thus, current supplied to the transistor TR21is controlled by using the switches 1801 to 1804 not to flow to thetransistor TR12.

In FIG. 2, the switch 1801 is turned ON while the switches 1802 to 1804are turned OFF. The switch 1801 is connected to a second power supply onthe high potential side (Vdd2). However, the switch 1801 may beconnected to a first power supply on the high potential side (Vdd1)connected to the load 1812 and the like. That is, a connection whichallows current to flow to the transistor TR11 but not to the transistorTR12 is used. Therefore, the switches 1802 and 1803 may be arrangeddifferently such that the switch 1802 is disposed between the sourceterminal of the transistor TR11 and the drain terminal of the transistorTR21. The loads 1812 and 1813 may have units for controlling current.Alternatively, current may be controlled by using the switch 1802 byremoving the switch 1801 and the second power supply on the highpotential side (Vdd2). In that case, the load 1812 is required to becapable of flowing current.

In this manner, a voltage Va1 between the gate and source of thetransistor TR11 is stored in the capacitor 1812. By turning OFF theswitch 1806, 1808 or the like as shown in FIG. 3, charge accumulated inthe capacitor 1812 is held.

Next, a compensation operation is performed by using the transistorsTR12 and TR21 as shown in FIGS. 4 and 5. Each switch is turned ON/OFF asshown in FIGS. 2 and 3. A voltage Va2 between the gate and source of thetransistor TR12 is stored in the capacitor 1813. In this manner, thecompensation operation terminates.

The compensation operation may be performed at least once before thenormal operation is performed. That is, the normal operation can beperformed as many times as needed as far as appropriate voltages areheld in the capacitors 1812 and 1813. However, the charges stored in thecapacitors 1812 and 1813 may eventually change due to a noise or a leakcurrent. In that case, a compensation operation is performed once againbefore the charges stored in the capacitors 1812 and 1813 changedrastically.

The normal operation is performed as shown in FIG. 6. That is to say,the switches 1801, 1804, 1806, 1808, 1809, and 1811 are turned OFF whilethe switches 1802, 1803, 1805, 1807, and 1810 are turned ON. In thismanner, the voltages Va1 and Va2 between the gate and source of eachtransistor can be changed according to the variation of thecharacteristics of the transistors TR11 and TR12, which can decrease theeffect of the variation. It should be noted in the normal operation thatthe voltage between the gate and source of each of the transistors TR11and TR12 may change according to the current flowing to each transistor.In that case, the voltage between the gate and source may not be Va1 andVa2. However, a value reflecting the variation of the characteristics isapplied to the gate terminal of the transistor, which decreases theeffect of the variation.

In the case where the terminal for outputting an output voltage Vo1 hasa high input impedance, the switch 1805 may be removed. The switch 1805may not be required according to the arrangements of the loads 1812 and1813.

By using such a differential circuit, a variety of circuits can beformed. By using a resistor or an active load circuit as the loads 1812and 1813, for example, a differential amplifier circuit can be formed.Further, by disposing a diodeconnected transistor (a transistor of whichgate terminal and drain terminal are connected) as the loads 1812 and1813, a part of an OTA (Operational Transconductance Amplifier) can beformed. By using these circuits in combination, it is possible to formsuch circuits as an operational amplifier, a sense amplifier, and acomparator.

An arrangement of the configuration of the differential amplifiercircuit using an active load circuit as the loads 1812 and 1813 isdescribed as an example.

First, a possible error is made small by performing the compensationoperation and the normal operation with close operating points to eachother.

Under the most typical operating condition of the differential amplifiercircuit, input voltages Vi1 and Vi2 have the same potentials. In thatcase, half of the current flowing through the transistor TR21 issupplied to the transistor TR11 and the other half is supplied to thetransistor TR12.

On the other hand, it is preferable that operating conditions such asoperating points are close in the compensation operation and the normaloperation. In order to make the operating points close, current for thecompensation operation may be half as much as that for the normaloperation. FIGS. 25 and 26 show an example of that case.

In FIG. 25, a transistor TR22 is added as a transistor that operates asa power supply. It is preferable that the transistors TR21 and TR22 arethe same in size. A bias voltage Vb is applied to the gate terminal ofeach transistor TR21 and TR22, and a switch 2501 is connected to thetransistor TR22 in series. By turning ON/OFF the switch 2501, current inthe compensation operation is made half as much as that in the normaloperation. It should be noted that the switch 2501 may be disposedanywhere as long as it can control current.

In FIG. 26, a transistor TR22 is added as a transistor that operates asa power supply. It is preferable that the transistors TR21 and TR22 arethe same in size. A bias voltage Vb is applied to the gate terminal ofthe transistor TR21, and the gate terminal of the transistor TR22 isapplied different voltage in the compensation operation and the normaloperation. Specifically, a power supply on the low potential side (Vss)is applied in the compensation operation so that the transistor TR22 isturned OFF. On the other hand, a bias voltage Vb is applied in thenormal operation. Thus, current in the compensation operation can behalf as much as that in the normal operation.

In this manner, by supplying different amount of current to a biasingtransistor as described above, operating points can be close in thecompensation operation and the normal operation. Error can be small whenoperating with close operating points.

Next, connections of the switches in the differential amplifier circuitusing an active load circuit are changed.

It has already been mentioned above that the arrangement of the switches1801 to 1804 in FIG. 2 can be changed. Now, the arrangement of theswitches 1801 to 1804 is changed in the differential amplifier circuitusing an active load circuit as the loads 1812 and 1813. FIG. 27 showsthe case in which the switch 1801 is removed.

The operation of each switch is as follows. In the case of flowingcurrent to the transistor TR11 and not to the transistor TR12, theswitches 1802 and 1803 are turned ON while the switch 1804 is turnedOFF. Then, the transistor 1813 is turned OFF because the voltage betweenthe gate and source of the transistor 1813 becomes 0 V. The transistor1812 is also turned OFF, however, current flows via the switch 1803 andthe switch 1802. In the case of flowing current to the transistor TR12and not to the transistor TR11, the switch 1802 is turned OFF and theswitch 1804 is turned ON, while the switch 1803 may be turned either ONor OFF. Then, current flows only to the transistor TR12. Last of all, inthe case of flowing current to both transistors TR11 and TR12, that isin the normal operation, the switch 1802 is turned ON and the switches1803 and 1804 are turned OFF.

Such arrangement of the switches as described above is viable. It shouldbe noted that the connections are not limited to this.

In this manner, a variety of circuits can be formed by applying theinvention to the differential circuit.

The description so far has been made on the case where the transistorsTR11 and TR12 are n-channel transistors. However, the invention can beapplied in the case where these transistors are p-channel transistors.FIG. 28 shows the case where the circuit in FIG. 1 uses p-channeltransistors as an example.

A reference voltage may be determined arbitrarily, therefore, a terminalwhich is applied the reference voltage may be connected to other wiring,node, or terminal. For example, terminals applied reference voltages Vx1and Vx2 may be connected to terminals that are applied input voltagesVi1 and Vi2 or the drain terminals of the transistors.

Embodiment Mode 2

In this embodiment mode, a source follower circuit and the configurationand operation thereof are described as an example of the analog circuitof the invention. The configuration of the source follower circuit ofthe invention is described with reference to FIG. 18.

In FIG. 18, the transistor TR1 is an n-channel transistor that amplifiescurrent. The transistor TR2 is an n-channel transistor that normallyoperates as a power supply and adjusts a bias voltage over the sourcefollower circuit. The capacitor 104 holds a voltage between the gate andsource of the transistor TR1. Further, denoted as 101 to 103 and 105 areswitches, which are preferably semiconductor elements such as atransistor. By controlling the switches 101 to 103 and 105, theconnections of the source follower circuit changes in the compensationoperation and the normal operation.

In FIG. 18, the drain terminal of the transistor TR1 is connected to thepower supply on the high potential side (Vdd). The source terminal ofthe transistor TR2 is connected to the power supply on the low potentialside (Vss). It is assumed for simplicity that the potential of the powersupply on the low potential side (Vss) is 0 V. A terminal 106 is thesource terminal of the transistor TR1, which is connected to the drainterminal of the transistor TR2 and also connected to an output terminal110 via the switch 105.

A terminal 107 is applied a reference voltage Vx and connected to thegate terminal of the transistor TR1 and one terminal of the capacitor104 via the switch 101. An input terminal 108 is applied an inputvoltage Vi and connected to the other terminal of the capacitor 104 viathe switch 102. The other terminal of the capacitor 104 is connected tothe source terminal 106 of the transistor TR1 via the switch 103. A gateterminal 109 of the transistor TR2 is applied a bias voltage Vb.

The operation of the source follower circuit shown in FIG. 18 isdescribed now.

The compensation operation is performed first. The switches 101 and 103are turned ON to be conductive while the switch 105 is turned OFF to benon-conductive. The gate terminal 109 of the transistor TR2 is appliedthe bias voltage Vb, therefore, current flows through the transistorTR2. At this time, the terminal 106 is connected to the terminal 107 viathe capacitor 104. The terminal 107 is applied the reference voltage Vx.Therefore, current flows between the terminals 107 and 106. When voltageat each end of the capacitor 104 becomes higher than the thresholdvoltage of the transistor TR1, the transistor TR1 turns ON, which allowscurrent to flow between the source and drain thereof. When a value ofcurrent flowing between the source and drain of the transistor TR2 and avalue of current flowing between the source and drain of the transistorTR1 become equal, current does not flow to the capacitor 104, which is asteady state.

At this time, a voltage required to flow the same amount of current tothe transistor TR1 as that flowing through the transistor TR2, that isthe voltage between the gate and source of the transistor TR1 is held inthe capacitor 104. Therefore, when current characteristic or size of thetransistor TR1 varies, the voltage between the gate and source of thetransistor TR1 varies accordingly. It is assumed that the voltagebetween the gate and source of the transistor TR1 at this time is Va.Then, the potential of the terminal 106 is lower than the referencevoltage Vx by Va.

Now that the circuit is in the steady state and current does not flowbetween the terminals 106 and 107, the switches 101 and 103 may beturned OFF without any problem. As a result, charge in the capacitor 104is held and the voltage at each end of the capacitor 104 does not changefollowing the law of conservation of electric charge.

In this manner, the compensation operation terminates. By thiscompensation operation, an appropriate voltage is stored in thecapacitor 104.

When current does not keep flowing to the output terminal 110 in thecompensation operation, that is when the input impedance of the outputterminal 110 is high enough, the switch 105 can be removed to connectthe terminal 106 and the output terminal 110 directly.

The compensation operation may be performed at least once before thenormal operation is performed. That is, the normal operation can beperformed as many times as needed as far as an appropriate voltage isheld in the capacitor 104. However, the charge stored in the capacitor104 may eventually change due to a noise or a leak current. In thatcase, a compensation operation is performed once again before the chargestored in the capacitor 104 changes drastically.

Next, the normal operation is performed. The switches 102 and 105 areturned ON and the switches 101 and 103 are turned OFF. The terminal 108is applied an input voltage Vi. Therefore, the gate terminal of thetransistor TR1 is applied a voltage in which a voltage Va of thecapacitor 104 is added to the input voltage Vi. When the circuit is in asteady state, a value of current flowing between the source and drain ofthe transistor TR2 and a value of current flowing between the source anddrain of the transistor TR1 become equal. The voltage between the gateand source of the transistor TR1 is Va.

Therefore, the potential of the terminal 106 is lower than the potentialof the gate terminal of the transistor TR1 by the voltage Va between thegate and source of the transistor TR1. The potential of the gateterminal of the transistor TR1 is higher than the input voltage Vi byVa. As described above, the potential of the terminal 106 becomes equalto the input voltage Vi. That is, the output voltage Vo becomes equal tothe input voltage Vi.

Therefore, the output voltage Vo is not dependent on the value of thereference voltage Vx. This means that the reference voltage Vx can haveany potential. That is, the reference voltage Vx can have an arbitrarypotential as long as the compensation operation is performed normally.It should be noted that the reference voltage Vx preferably has apotential which allows the transistors TR1 and TR2 to operate in asaturation region. This is because the transistors in the sourcefollower circuit typically operate in a saturation region.

As the reference voltage Vx can have an arbitrary potential, theterminal 107 may be connected to other wiring, node, or terminal. Forexample, the terminal 107 may be connected to the input terminal 108. Atthis lime, since the reference voltage Vx can have an arbitrarypotential, the input voltage Vi in the compensation operation can havean arbitrary potential as well. Therefore, the input voltage Vi may havedifferent potentials in the compensation operation and the normaloperation.

Similarly, the terminal 107 may be connected to any of the power supplyon the high potential side (Vdd), the drain terminal of the transistorTR1, the output terminal 110, or the terminal 109. In this manner, theterminal 107 can be connected arbitrarily.

As well as being not dependent on the reference voltage Vx, the outputvoltage Vo is not dependent on the voltage Va between the gate andsource of the transistor TR1. This means that the voltage Va between thegate and source of the transistor TR1 can have an arbitrary potential.That is, the output voltage Vo is not affected when the currentcharacteristics (mobility, threshold voltage and the like) or the size(gate length L and gate width W) of the transistor TR1 vary.

The output voltage Vo is not dependent on the current flowing betweenthe source and drain of the transistor TR1 or TR2. That is to say, theoutput voltage Vo is not dependent on the bias voltage Vb that isapplied to the gate terminal 109 of the transistor TR2. Further, theoutput voltage Vo is not also dependent on the current characteristics(mobility, threshold voltage and the like) or the size (gate length Land gate width W) of the transistor TR2.

In this manner, the gate terminal of the transistor TR1 in the normaloperation is not applied the input voltage Vi as it is, but the voltagein which the voltage stored in the capacitor 104 is added to the inputvoltage Vi. The voltage stored in the capacitor 104 has a potentialcorresponding to the state of the condition. That is, the voltage storedin the capacitor 104 changes according to the variation of the currentcharacteristics and sizes of the transistors TR1 and TR2. Accordingly,an effect of variation of the transistors TR1 and TR2 can be decreased.

FIG. 18 shows the case where the transistors TR1 and TR2 are n-channeltransistors. However, the invention can be applied in the case wherethese transistors are p-channel transistors. FIG. 13 shows the sourcefollower circuit using p-channel transistors as the transistors TR1 andTR2. The transistor TR1 amplifies current. The transistor TR2 normallyoperates as a power supply and adjusts a bias voltage over the sourcefollower circuit. Denoted as 104 is a capacitor which holds a voltagebetween the gate and source of the transistor TR1. It should be notedthat the detailed description of the operation and configuration areomitted here since they are the same in the case of using n-channeltransistors.

In FIGS. 18 and 13, the transistor TR2 that operates as a power supplyand adjusts a bias voltage over the source follower circuit is disposed.However, the transistor TR2 may not necessarily be disposed. Thiscorresponds to the case where the transistor TR2 has a current value ofzero.

FIG. 7 shows a circuit diagram of the source follower circuit withoutthe transistor TR2 20 shown in FIG. 18. A switch 701 is connectedbetween the terminal 106 and the power supply on the low potential side(Vss). The switch 701 can turn ON the transistor TR1 in the compensationoperation. Therefore, the switch 701 may be disposed at other place aslong as it can turn ON the transistor TR1 in the compensation operation.Otherwise, the switch 701 may not be provided.

Next, the operation of the source follower circuit without thetransistor TR2 shown in FIG. 7 is described.

The compensation operation is performed first. The compensationoperation can be divided into two stages. In the first stage, thetransistor TR1 is turned ON. Thereafter in the second stage, the voltagebetween the gate and source of the transistor TR1 and the thresholdvoltage of the transistor TR1 are made almost equal.

In the circuit shown in FIG. 18, the compensation operation does nothave to be divided into two stages, however, connection of the circuitand the like are required to be changed according to each stage of thecompensation operation in the circuit in FIG. 7.

In the first stage of the compensation operation, the transistor TR1 isturned ON by turning ON the switches 101, 103, and 701 and turning OFFthe switches 102 and 105. Therefore, the voltage between the gate andsource of the transistor TR1 at this time is higher than the thresholdvoltage of the transistor TR1.

The operation method in the first stage is not limited to theaforementioned method as long as the transistor TR1 can be turned ON.For example, the switch 701 is removed so that the terminal 106 and thepower supply on the low potential side (Vss) are not connected. Theswitch 102 is turned ON and the reference voltage Vx and the inputvoltage Vi are controlled to turn ON the transistor TR1.

Next, in the second stage of the compensation operation, the switches101 and 103 are turned ON and the switches 102, 105 and 701 are turnedOFF. Thus, the source terminal of the transistor TR1 is connected onlyto the capacitor 104. Then, current flows between the source and drainof the transistor TR1 when turned ON. The current flows into thecapacitor 104, and the charge stored in the capacitor 104 is released asa result. The charge keeps being released until the transistor TR1 isturned OFF, that is when the voltage between the gate and source of thetransistor TR1 becomes equal to the threshold voltage of the transistorTR1. When the voltage between the gate and source of the transistor TR1becomes equal to the threshold voltage of the transistor TR1, currenthardly flows to the transistor TR1 and the capacitor 104.

Now that current hardly flows in the circuit, the switches 101 and 103can be turned OFF without any problem since no current flows between theterminals 106 and 107. As a result, the charge in the capacitor 104 isheld and the potential at each end of the capacitor 104 does not changefollowing the law of conservation of electric charge.

In this manner, the compensation operation terminates. By thiscompensation operation, the threshold voltage of the transistor TR1 isheld in the capacitor 104.

The compensation operation is not necessarily performed until thevoltage of the capacitor 104 and the threshold voltage of the transistorTR1 become equal. It may be performed until the voltage of the capacitor104 and the threshold voltage of the transistor TR1 become almost equal.

Subsequently, the normal operation is performed. The switches 102 and105 are turned ON and the switches 101, 103, and 701 are turned OFF. Theterminal 108 is applied an input voltage Vi. Therefore, the gateterminal of the transistor TR1 is applied a voltage in which the voltageof the capacitor 104, that is the threshold voltage of the transistorTR1 is added to the input voltage Vi. When the steady state is obtained,current hardly flows between the source and drain of the transistor TR1.The voltage between the gate and source of the transistor TR1 at thattime is almost equal to the threshold voltage of the transistor TR1.

Therefore, the potential of the terminal 106 is lower than the potentialof the gate terminal of the transistor TR1 by the threshold voltage ofthe transistor TR1. The potential of the gate terminal of the transistorTR1 is higher than the input voltage Vi by the voltage of the capacitor104, that is the threshold voltage of the transistor TR1. As describedabove, the potential of the terminal 106 is equal to the input voltageVi. That is, the output voltage Vo becomes equal to the input voltageVi.

The transistor TR2 that operates as a power supply is not provided inFIG. 7, however, it may be provided as well. The circuit diagramincluding the transistor TR2 is shown in FIG. 15. The operation thereofis the same as in FIG. 7 in respect of the compensation operation inwhich the threshold voltage is held in the capacitor 104. In the normaloperation, however, the switch 701 is required to be turned ON since thetransistor TR2 has to operate as a power supply.

Note that a capacitor may be connected to the transistor TR2 to storethe threshold voltage thereof to compensate for variation of thetransistor TR2.

In this manner, the invention can be similarly applied to the circuitwithout the transistor TR2. Therefore, it is also the same in thecircuit without the transistor TR2 that the reference voltage Vx has anarbitrary value and variation of the current characteristics (mobility,threshold voltage and the like) or the size (gate length L and gatewidth W) of the transistor TR1 does not affect. The transistor TR1 is ann-channel transistor in FIG. 7, however, it may be a p-channeltransistor as well.

By using both an n-channel transistor TR1 and a p-channel transistor TR1as amplifier transistors in combination, a push-pull amplifier may beformed as well. The circuit diagram in that case is shown in FIG. 14.The p-channel transistor TR1p is connected to the power supply on thelow potential side (Vss) and a capacitor 104p is connected between thegate and source thereof. The n-channel transistor TR1n is connected tothe power supply on the high potential side (Vdd) and the capacitor 104nis connected between the gate and source thereof. The operation and thelike thereof are omitted here since it is similar to the case of FIG. 7.

The capacitor may store the threshold voltage of the transistor insteadof the voltage between the gate and source of the transistor in thedifferential circuit as well as in the source follower circuit as shownin FIG. 15. For example, when applying the aforementioned idea to thecircuit in FIG. 1, it is required that a switch is disposed between thesource terminal of the transistor TR11 and the drain terminal of thetransistor TR21, and between the source terminal of the transistor TR12and the drain terminal of the transistor TR21.

Described in this embodiment mode is the case of applying the inventionto the source follower circuit, however, it can also be applied to acascode circuit which is quite similar to the source follower circuit inconfiguration. The cascode circuit is different from the source followercircuit in the following respect. Taking FIG. 21 as an example, the gateterminal 4309 of the transistor TR2 is an input terminal, the gateterminal 4308 of the transistor TR1 is a terminal for applying a biasvoltage, such a load as a resistor is disposed between the drainterminal of the transistor TR1 and the power supply on the highpotential side (Vdd), and a node of the load and the drain terminal ofthe transistor TR1 is an output terminal.

FIG. 16 shows a circuit diagram of the cascode circuit to which theinvention is applied. A load 1601 is disposed between the drain terminalof the transistor TR1 and the power supply on the high potential side(Vdd). It should be noted that the transistors TR1 and TR2 are n-channeltransistors in FIG. 16, however, it is needless to say that they may bep-channel transistors as well. The operation and the like of theaforementioned cascode circuit are similar to the source followercircuit, therefore, the description is omitted here.

Finally, a method for decreasing power consumption of the circuit isdescribed. In the analog circuit, current often keeps flowing even inthe steady state. In the source follower circuit, for example, currentnormally keeps flowing from the transistor TR1 to the transistor TR2even in the steady state, which consumes much power. Therefore, it ispossible to decrease power consumption by interrupting the currentflowing in the steady state. FIG. 17 shows a circuit based on FIG. 18that is arranged to decrease power consumption. In the circuit in FIG.17, a switch 1701 is disposed between the power supply on the highpotential side (Vdd) and the drain terminal of the transistor TR1. Bycontrolling the switch 1701, current flowing from the transistor TR1 tothe transistor TR2 in the steady slate can be interrupted. Note that theswitch 1701 may be disposed anywhere as long as it can interrupt theflowing current. Further, the flowing current may be interrupted withoutproviding the switch 1701. For example, the voltage Vb of the gateterminal 109 of the transistor TR2 may be controlled so as not to flowcurrent to the transistor TR2. Similarly, the potential of the gateterminal of the transistor TR1 may be controlled so as not to flowcurrent.

It should be noted that the aforementioned idea that current which keepsflowing in the steady state is interrupted in order to decrease powerconsumption may be applied to a differential circuit as well.

The description in Embodiment Mode 1 can be applied to this embodimentmode, and the description made in this embodiment mode can be applied toEmbodiment Mode 1 as well.

Embodiment Mode 3

In the aforementioned Embodiment Modes 1 and 2, the source followercircuit and the differential circuit to which the invention is appliedhave been described. By using these circuits in combination, it can beapplied to a variety of circuits. In this embodiment mode, anoperational amplifier to which the invention is applied is described asan example.

It should be noted that the operational amplifier has various circuitconfigurations. Therefore, the circuit configuration of the operationalamplifier is not limited to this embodiment mode. The invention can beapplied to operational amplifiers of various configurations.

As the simplest configuration of the operational amplifier, adifferential amplifier circuit is used in combination with a sourcefollower circuit. As shown in FIG. 29, the circuit in FIG. 1 is used asthe differential circuit, an active circuit is used as a load of thedifferential circuit, and the circuit in FIG. 18 is used as the sourcefollower circuit. A region 2901 surrounded by a dotted line correspondsto the source follower circuit. Signals are inputted from a positiveinput terminal 2901 and a negative input terminal 2902 and outputtedfrom an output terminal 2903. By controlling the voltage to apply to abias terminal 2904, current to flow as a bias is controlled. Thecompensation operation and the normal operation of each portion areswitched by controlling a timing to input signals to terminals 2905 to2909. It should be noted that by changing connection to the terminals2905 to 2909 and the like, the compensation operation can be performedsimultaneously in a plurality of portions in the circuit.

An operational amplifier including a push-pull buffer in the outputstage is shown in FIG. 30. The circuit in FIG. 14 is used as a push-pullsource follower circuit. A region 3011 surrounded by a dolled linecorresponds to the push-pull source follower circuit. In FIG. 30,signals are inputted from a positive input terminal 3001 and a negativeinput terminal 3002 and outputted from an output terminal 3003. Bycontrolling the voltage to apply to a bias terminal 3004, current toflow as a bias is controlled. The compensation operation and the normaloperation of each portion are switched by controlling a timing to inputsignals to terminals 3005 to 3010. It should be noted that by changingconnection to the terminals 3005 to 3010 and the like, the compensationoperation can be performed simultaneously in a plurality of portions inthe circuit.

Subsequently, an operational amplifier having two amplification stagesis shown in FIG. 31. A common source amplifier circuit is used as asecond amplification stage. A region 3111 surrounded by a dotted linecorresponds to the common source amplifier circuit. In FIG. 31, signalsare inputted from a positive input terminal 3101 and a negative inputterminal 3102, and outputted from an output terminal 3103. Current to besupplied as a bias is controlled by controlling a voltage to apply to abias terminal 3104. By controlling a timing to input a signal toterminals 3105 to 3109, the compensation operation and the normaloperation of each portion are switched. Note that compensation operationcan be performed in a plurality of circuit portions at the same time bychanging the connection to the terminals 3105 to 3109 and the like.

A capacitor 3110 is provided for phase compensation. It may be disposedat other place or a resistor may be disposed in series. An additionalsource follower circuit may be disposed next to the second amplificationstage.

Here, a common source amplifier circuit is briefly described. FIG. 32shows a common source amplifier circuit to which the invention isapplied.

It should be noted that in a conventional common source amplifiercircuit, the drain terminal of a transistor TR4 for supplying a biascurrent and the drain terminal of an amplifier transistor TR3 areconnected and the node thereof is an output terminal. The sourceterminals of the transistors TR3 and TR4 are grounded, therefore,polarities of both transistors are opposite. The gate terminal of thetransistor TR4 is applied a bias voltage and the gate terminal of thetransistor TR3 is applied an input voltage.

On the other hand, switches 3201 to 3203 and 3205 and a capacitor 3204are added in the common source amplifier circuit in FIG. 32. In the casewhere an output terminal 3210 has a high input impedance, the switch3205 may be removed to connect the drain terminal of the transistor TR3and the output terminal 3210.

Subsequently, the operation of the common source amplifier circuit inFIG. 32 is described with reference to FIGS. 33 and 34. First, thecompensation operation is performed. As shown in FIG. 33, the switches3203 and 3202 are turned ON and the switches 3201 and 3205 are turnedOFF. Then, the voltage Va between the gate and source of the transistorTR3 is stored in the capacitor 3204.

Thereafter, the normal operation is performed. As shown in FIG. 34, theswitches 3201 and 3205 are turned ON and the switches 3202 and 3203 areturned OFF. Then, an input voltage Vi is applied to an input terminal3208. Then, the voltage in which the voltage Va stored in the capacitor3204 is added to the input voltage Vi is applied to the gate terminal ofthe transistor TR3. The voltage Va stored in the capacitor 3204 has apotential corresponding to the current characteristics of the transistorTR3. Therefore, effect of variation of the transistor TR3 can bedecreased.

It should be noted that the compensation operation may be performed atleast once as in the source follower circuit.

The threshold voltage of the transistor may be stored in the capacitor3204 instead as in FIG. 7 and the like.

In the case of using the aforementioned common source amplifier circuitas part of an operational amplifier, a capacitor or a resistor may bedisposed in the common source amplifier circuit for the phasecompensation of the operational amplifier. As an example, FIG. 35 showsa circuit diagram in which a capacitor 3501 is disposed between theinput terminal 3208 and the drain terminal of the transistor TR3. Notethat any element may be disposed anywhere as long as the phasecompensation of the operational amplifier can be performed.

It should be noted that the description made in Embodiment Modes 1 and 2can be applied to this embodiment mode. For example, the timing andfrequency to perform the compensation operation is similar in thisembodiment mode to Embodiment Modes 1 and 2.

Further, the reference voltage may be determined arbitrarily, therefore,a terminal to which is applied the reference voltage may be connected toother wiring, node, or terminal.

Further, the capacitor may hold the threshold voltage of the transistorinstead of the voltage between the gate and source thereof.

Further, current that keeps flowing in the steady state may beinterrupted for decreasing power consumption in this embodiment mode aswell.

Further, the transistors used in this embodiment mode are mainlyn-channel transistors, however, they may be p-channel transistors.

It should be noted that the invention is applied to the operationalamplifier in this embodiment mode, however, it can be applied to suchcircuits as an OTA (Operational Transconductance Amplifier), a senseamplifier, or a comparator. Moreover, the invention can be applied to acircuit in which transistors are cascaded.

This embodiment mode can be freely combined with Embodiment Modes 1 and2.

Embodiment Mode 4

In this embodiment mode, a method for saving time in using an electriccircuit to which the invention is applied is described.

As described above, the circuit of the invention has two operatingstates. One is the compensation operation and the other is the normaloperation. The compensation operation is not required to be performedfrequently, however, it is required to be performed at least once beforethe normal operation is performed.

In the case where one circuit (for example, a source follower circuit)is disposed between a pair of input terminal and output terminal, thecompensation operation may be performed at the following timings.

As one of the timings, the compensation operation is definitelyperformed before the normal operation is performed. For example, acertain period in which signals are inputted and outputted is dividedinto two periods. The compensation operation is performed in the firstperiod and the normal operation is performed in the second period.

As a second timing, the compensation operation is performed in a periodin which signals are not inputted or outputted and thereafter the normaloperation is performed as many times as required.

As another timing, the normal operation is performed while thecompensation operation is performed. In that case, only one circuitdisposed between a pair of input terminal and output terminal is notenough to perform the compensation operation and the normal operation atthe same time. Therefore, two circuits or more are disposed in parallelbetween the pair of input terminal and output terminal. Thus, the normaloperation can be performed while the compensation operation is performedby controlling the operation of each disposed circuit.

FIG. 8 shows two source follower circuits disposed in parallel between apair of input terminal and output terminal. A circuit 3603 is disposedbetween an input terminal 3601 and an output terminal 3602. The circuit3603 includes source follower circuits 3604 and 3605. One of the sourcefollower circuits performs the normal operation to output a signal tothe output terminal 3602 while the other source follower circuitperforms the compensation operation. A signal inputted to the terminal3606 switches which Source follower circuit to perform which operation.In FIG. 8, the source follower circuit 3604 performs the compensationoperation when the terminal 3606 inputs a H signal, while the sourcefollower circuit 3605 performs the compensation operation when theterminal 3606 inputs an L signal.

In this manner, the normal operation can be performed while thecompensation operation is performed. By performing two operations at thesame time, the operations can be performed efficiently without wastingtime, which makes it possible to take enough time for each operation.Therefore, the compensation operation can be performed until the steadystate is obtained, which enables an accurate compensation.

Note that the timing to perform the compensation operation is notlimited to the abovementioned one.

FIG. 8 shows the case of using source follower circuits, however, twocircuits or more can be disposed between a pair of input terminal andoutput terminal in other circuits such as a differential circuit and anoperational amplifier as well.

It should be noted that this embodiment mode can be freely combined withEmbodiment Modes 1 to 3.

Embodiment Mode 5

In this embodiment mode, configurations and operations of a displaydevice, a signal driver circuit and the like are described. The circuitof the invention can be applied to a part of the signal driver circuit.

The display device shown in FIG. 9 includes pixels 3701, a gate drivercircuit 3702, and a signal driver circuit 3710. The gate driver circuit3702 outputs selection signals to the pixels 3701 sequentially. Thesignal driver circuit 3710 outputs video signals to the pixels 3701sequentially. The pixels 3701 display an image by controlling lightaccording to the video signals. The video signals inputted from thesignal driver circuit 3710 to the pixels 3701 are often voltage. Thatis, display elements or elements for controlling the display elementsdisposed in the pixels are often the ones that change their states inaccordance with the inputted video signals (voltage) from the signaldriver circuit 3710. The display elements disposed in the pixels includea liquid crystal (LCD), an organic EL, an FED (Field Emission Display)and the like.

Note that a plurality of the gate driver circuits 3702 or the signaldriver circuits 3710 may be disposed.

The configuration of the signal driver circuit 3710 can be divided intoa plurality of portions. It can be briefly divided as an example into ashift register 3703, a first latch circuit 3704, a second latch circuit3705, a digital/analog converter circuit 3706, and a buffer circuit(amplifier circuit) 3707.

The operation of the signal driver circuit 3710 is briefly described.The shift register 3703 includes a plurality of columns of flip-flopcircuit (FF) and the like, to which a clock signal (S-CLK), a startpulse (SP), an inverted clock signal (S-CLKb) are inputted. Samplingpulses are outputted sequentially according to the timing of thesesignals.

The sampling pulses outputted from the shift register 3703 are inputtedto the first latch circuit 3704. The first latch circuit 3704 isinputted a video signal from a video signal line 3708 which is stored ineach column in accordance with the timing at which the sampling pulsesare inputted. It should be noted that the video signal has a digitalvalue when the digital/analog converter circuit 3706 is provided.

When video signals are held up to the last column in the first latchcircuit 3704, a latch pulse is inputted form a latch control line 3709in a horizontal retrace period. The video signals held in the firstlatch circuit 3704 are transferred to the second latch circuit 3705 allat once. After that, one row of the video signals held in the secondlatch circuit 3705 are inputted to the digital/analog converter circuit3706 at a time. The signals outputted from the digital/analog convertercircuit 3706 are inputted to the buffer circuit (amplifier circuit)3707. Then, the signals are inputted from the buffer circuit (amplifiercircuit) 3707 to the pixels 3701.

While the video signals held in the second latch circuit 3705 areinputted to the digital/analog converter circuit 3706 and to the pixels3701, a sampling pulse is outputted from the shift register 3703 again.That is to say, two operations are performed at the same time.Accordingly, line sequential driving can be performed. Theaforementioned operations are repeated further.

The invention can be applied to the buffer circuit (amplifier circuit)3707 in the signal driver circuit 3710 in which the aforementionedoperations are performed. The buffer circuit (amplifier circuit) 3707 iscapable of supplying a large amount of current to the pixels 3711. Thatis to say, the buffer circuit (amplifier circuit) 3707 can convertimpedance. A source follower circuit, a differential amplifier circuit,or an operational amplifier and the like can be used as this buffercircuit (amplifier circuit) 3707. In the case of using the differentialamplifier circuit or operational amplifier, it can operate as a voltagefollower circuit by connecting an output terminal to a negative inputterminal to feed back the signals.

By disposing a plurality of source follower circuits, differentialamplifier circuits, or operational amplifiers as shown in FIG. 8, thecompensation operation and the normal operation can be performed at thesame time.

In the case where the first latch circuit 3704 and the second latchcircuit 3705 are capable of storing analog values, the digital/analogconverter circuit 3706 can often be removed. Further, in the case wherethe data of binary digits, that is digital values are outputted to thepixels 3701, the digital/analog converter circuit 3706 can also beremoved. The digital/analog converter circuit 3706 may include a gammacorrection circuit. In this manner, the signal driver circuit 3710 mayhave a variety of configurations and not limited to FIG. 9.

FIG. 10 shows the signal driver circuit 3710 in the case where the firstlatch circuit 3704 and the second latch circuit 3705 are capable ofstoring analog values. A video signal having analog value is inputtedfrom the video signal line 3708. FIG. 11 shows an example of each column3801 of the first latch circuit 3704 and the second latch circuit 3705.The example of each column 3801 includes one column of the first latchcircuit 3704 and one column of the second latch circuit 3705. The onecolumn of the first latch circuit 3704 includes a capacitor 3901 and abuffer circuit (amplifier circuit) 3902. The one column of the secondlatch circuit 3705 includes a capacitor 3903 and a buffer circuit(amplifier circuit) 3904.

Each column 3801 of the first latch circuit 3704 and the second latchcircuit 3705 operates as follows. First, an analog video signal isinputted from the video signal line 3708 to the capacitor 3901 to bestored therein. The data stored in the capacitor 3901 is transferred tothe capacitor 3903 by the signal from the latch control line 3709. Atthis time, the buffer circuit (amplifier circuit) 3902 converts theimpedance. Therefore, the buffer circuit (amplifier circuit) 3902 can beremoved by controlling the capacitances of the capacitors 3901 and 3902.The signals stored in the capacitor 3903 are outputted to the pixels viathe buffer circuit (amplifier circuit) 3904.

A source follower circuit, a differential amplifier circuit, operationalamplifier and the like can be used as these buffer circuits (amplifiercircuits) 3902 and 3904. FIG. 12 shows a circuit diagram of the buffercircuit (amplifier circuit) using source follower circuits as anexample. As shown in FIG. 8, a plurality of buffer circuits (amplifiercircuits) may be disposed so that the compensation operation and thenormal operation can be performed at the same time.

Note that this embodiment mode can be freely combined with EmbodimentModes 1 to 4.

Embodiment Mode 6

In this embodiment mode, a layout of an electric circuit using theinvention is described.

In this embodiment mode, the layout of a source follower circuit towhich the invention is applied is described. FIG. 19 shows a circuitdiagram of a source follower circuit of which layout is describedsimilarly to FIG. 18.

In FIG. 19, the capacitor 104 is formed as a MOS capacitor. That is,when the MOS capacitor is considered as a transistor, the sourceterminal and the drain terminal thereof are connected and the nodethereof becomes one terminal of the capacitor and the gate terminalthereof becomes the other terminal of the capacitor. Such a capacitorformed in this manner can have a large capacitance. It should be notedthat the polarity of the capacitor 104 as a transistor is preferably thesame as that of the transistor TR1. This is because the MOS capacitor asa transistor in this case is required to be ON since the capacitancebecomes zero when it is OFF. Therefore, the capacitor 104 is required tohave the same polarity as the transistor TR1 in order to be ON.

FIG. 20 shows the layout of the source follower circuit of FIG. 19. Agate-insulating layer is formed on a semiconductor layer 4201 formed bypolycrystalline silicon and the like, and a gate wirings (first wirings)4202 formed thereon correspond to the transistors. Aninterlayer-insulating layer is formed on the gate wirings (firstwirings) 4202 and second wirings 4204 is formed thereon. The secondwirings 4204 and the semiconductor layer 4201, and the second wirings4204 and the gate wirings (first wirings) 4202 are connected by openingcontacts 4203.

With a known technology by applying such a layout as shown in FIG. 20,the electric circuit of the invention can be achieved.

It should be noted that the transistors TR1 and TR2 normally operate ina saturation region. In an ideal transistor, current flowing between thesource and drain thereof does not change in a saturation region evenwhen the voltage between the source and drain changes. In fact, however,current flowing between the source and drain of the transistor changeseven in a saturation region because of such effect as the kink effect orEarly effect, which causes an error. In order to decrease the kinkeffect and Early effect, the gate length L of the transistors TR1 andTR2 are designed long in FIG. 20. It should be noted that the kinkeffect and Early effect can be decreased by using other methods, forexample, by additionally connecting a transistor in series to thetransistors TR1 and TR2. Such methods can be applied to the invention aswell.

Ideally, the voltage in the capacitor 104 does not change between thecompensation operation and the normal operation. In fact, however, thevoltage in the capacitor 104 is divided because of a parasiticcapacitance (gate capacitance) of a transistor (the transistor TR1 here)of which gate terminal is connected to the capacitor 104. As a result,the voltage of the capacitor 104 slightly changes between thecompensation operation and the normal operation, which causes an error.In order to reduce the error, the capacitor 104 is required to have asufficiently larger capacitance than the parasitic capacitance (gatecapacitance) of the transistor of which gate terminal is connected tothe capacitor 104. Specifically, the capacitance of the capacitor 104 ispreferably five times as large as the parasitic capacitance (gatecapacitance) of the transistor of which gate terminal is connected tothe capacitor 104 or more.

It should be noted that this embodiment can be freely combined withEmbodiment Modes 1 to 5.

Embodiment Mode 7

An electronic device using the invention includes a video camera, adigital camera, a goggle display (head mounted display), a navigationsystem, an audio reproducing device (such as car audio system and audiocomponent system), a notebook personal computer, a game machine, aportable information terminal (such as mobile computer, mobiletelephone, portable game machine, and electronic book) and an imagereproducing device provided with a recording medium (specifically, adevice provided with a display capable of reproducing the recordingmedium such as a digital versatile disc (DVD) and displaying an imagethereof). FIG. 36 shows specific examples thereof.

FIG. 36(A) illustrates a display device including a housing 13001, asupport base 13002, a display portion 13003, speaker portions 13004, avideo input terminal 13005 and the like. The invention can be applied toan electric circuit forming the display portion 13003. According to theinvention, the display device illustrated in FIG. 36(A) can be formed.The display portion 13003 can be applied to an organic EL display, aliquid crystal display and the like. The display device includes theentire display devices for displaying information, such as a personalcomputer, a receiver of TV broadcasting and an advertising display.

FIG. 36(B) illustrates a digital still camera including a body 13101, adisplay portion 13102, an image receiving portion 13013, operating keys13104, an external connection port 13105, a shutter 13106 and the like.The invention can be applied to an electric circuit forming the displayportion 13102. According to the invention, the digital still cameraillustrated in FIG. 36(B) can be formed.

FIG. 36(C) illustrates a notebook personal computer including a body13201, a housing 13202, a display portion 13203, a keyboard 13204, anexternal connection port 13205, a pointing mouse 13206 and the like. Theinvention can be applied to an electric circuit forming the displayportion 13203. According to the invention, the display deviceillustrated in FIG. 36(C) can be formed.

FIG. 36(D) illustrates a mobile computer including a body 13301, adisplay portion 13302, a switch 13303, operating keys 13304, an infraredport 13305 and the like. The invention can be applied to an electriccircuit forming the display portion 13302. According to the invention,the mobile computer illustrated in FIG. 36(D) can be formed.

FIG. 36(E) illustrates a portable image reproducing device provided witha recording medium (specifically, a DVD reproducing device) including abody 13401, a housing 13402, a display portion A13403, a display portionB13404, a recording medium (such as a DVD) reading portion 13405, anoperating key 13406, a speaker portion 13407 and the like. The displayportion A13403 mainly displays image data while the display portionB13404 mainly displays text data. The invention can be applied toelectric circuits forming both of the display portions A13403 andB13404. It should be noted that the image reproducing device providedwith a recording medium includes a domestic game machine and the like.According to the invention, the DVD reproducing device illustrated inFIG. 36(E) can be formed.

FIG. 36(F) illustrates a goggle display (head mounted display) includinga body 3501, a display portion 13502, an arm portion 13503 and the like.The invention can be applied to an electric circuit forming the displayportion 13502. According to the invention, the goggle displayillustrated in FIG. 36(F) can be formed.

FIG. 36(G) illustrates a video camera including a body 13601, a displayportion 13602, a housing 13603, an external connection port 13604, aremote control receiving portion 13605, an image receiving portion13606, a battery 13607, an audio input portion 13608, operating keys13609 and the like. The invention can be applied to an electric circuitforming the display portion 13602. According to the invention, the videocamera illustrated in FIG. 36G can be formed.

FIG. 36(H) illustrates a portable phone including a body 13701, ahousing 13702, a display portion 13703, an audio input portion 13704, anaudio output portion 13705, operating keys 13706, an external connectionport 13707, an antenna 13708 and the like. The invention can be appliedto an electric circuit forming the display portion 13703. It should benoted that the power consumption of the display portion 13703 in theportable phone can be suppressed by displaying white text on blackbackground. According to the invention, the portable phone illustratedin FIG. 36(H) can be formed.

It should be noted that the light including the outputted image data canbe applied to a front or rear projection system by expanding it using alens and the like for projection provided that the display material hasa higher light-emitting luminance in the future.

Furthermore, the aforementioned electric devices are more likely todisplay information distributed through telecommunication lines such asinternet, CATV (cable TV) and the like, in particular moving image data.As the light-emitting material has a quite fast response, alight-emitting device is preferable for displaying moving images.

The light-emitting device consumes power in a light-emitting portion,therefore, it is preferable that the light emitting portion is as smallas possible when displaying the data. Therefore, in the case of usingthe light-emitting device in a display portion such as a portableinformation terminal, in particular a portable phone and an audioreproducing device that mainly display text data, it is preferable toform the text data with the light emitting portions on a non-lightemitting portion as a background.

As described above, the application range of the invention is quite wideand the invention can be applied to a variety of electronic devices. Theelectric devices described in this embodiment mode can employ anyelectric circuit or semiconductor device described in Embodiment Modes 1to 6.

In the invention, the gate terminal of the transistor is applied voltagein which voltage stored in the capacitor is added to the input voltage,not just the input voltage only. The voltage stored in the capacitor hasa potential corresponding to the current characteristics and the size ofthe transistor. Therefore, when the current characteristics and the sizeof the transistor vary, the voltage stored in the capacitor changesaccordingly. As a result, an effect of variation of the transistor canbe decreased.

An operation to store voltage in a capacitor, that is the compensationoperation may be performed at least once. Then, the effect of variationin characteristics of the transistor can be decreased in the subsequentnormal operation. Therefore, drive timing is not made complicated and asimple operation can be obtained.

Moreover, as the numbers of capacitors and switches are small, only asmall area is occupied for the layout. As a result, the manufacturingyield does not easily drop and downsizing of a device can be achieved.

1. An analog circuit comprising a first transistor, a first capacitor, afirst switch, a first terminal, a second terminal, a second transistor,a second capacitor, a second switch, a third terminal, and a fourthterminal, wherein a gate terminal of the first transistor and oneterminal of the first capacitor are electrically connected; wherein agate terminal of the second transistor and one terminal of the secondcapacitor are electrically connected; wherein a source terminal of thefirst transistor and a source terminal of the second transistor areelectrically connected; the first terminal and one terminal of the firstcapacitor are electrically connected via the first switch; and the thirdterminal and one terminal of the second capacitor are electricallyconnected via the second switch, comprising a unit for electricallyconnecting the other terminal of the first capacitor and either of thesecond terminal or a source terminal of the first transistor; and a unitfor electrically connecting the other terminal of the second capacitorand either of the fourth terminal or a source terminal of the secondtransistor.
 2. The analog circuit according to claim 1, comprising aunit for supplying current, wherein the source terminal of the firsttransistor and the unit for supplying current are electricallyconnected.
 3. The analog circuit according to claim 1, comprising: aunit for interrupting current supplied to the first transistor; and aunit for interrupting current supplied to the second transistor.
 4. Theanalog circuit according to claim 1, wherein the first terminal and thesecond terminal are electrically connected; and wherein the thirdterminal and the fourth terminal are electrically connected.
 5. Theanalog circuit according to claim 1, which forms any one of adifferential amplifier circuit, an operational amplifier, and a signaldriver circuit.
 6. A display device provided with the analog circuit setforth in claim
 1. 7. An electronic device provided with a display deviceset forth in claim 6.